It is well known in the hydraulic art to have a fluid system wherein the delivery flow rate of the pump is controlled so as to provide the needed flow to an actuator at a pressure higher by a fixed value than the pressure required to move the load. This type of system is typically called a load sensing system. Likewise, it is well-known to provide a pressure compensating valve in each pressure line that supplies fluid to respective directional control valves in order to maintain a predetermined differential pressure across an orifice in the directional control valve regardless of variations in the load. By maintaining a predetermined differential pressure across the orifice of the directional control valve, the volume of fluid passing therethrough remains constant for a given orifice size irrespective of changing load conditions. This type of pressure compensating valve is normally referred to as a positive load pressure compensating valve. The term "positive load" refers to a system wherein the load is a resisting type of load. It is also well known to use a negative load pressure compensating valve in the exhaust flow line to control the rate of fluid flow across the directional control valve during conditions in which the load is an aiding type load. In an aiding type load, the load is attempting to exhaust the fluid from the actuator faster than the fluid is being introduced thereto. This operating condition is referred to as a "negative load".
The different types of pressure compensating valves noted above have a valving element therein which serves as a variable restriction in order to control the fluid flow thereacross to maintain the constant differential pressure across the orifice in the directional control valve. It is well known that as the pressure drop across the valving element increases, a force, normally referred to as a flow force, is created which acts on the valving element adversely affecting its operation. In most cases involving pressure compensating valves, this unwanted force acts to prematurely cause the valving element to further restrict the flow thereacross. This unwanted restriction results in the differential pressure across the directional control valve being changed. Normally this change is in the direction of reducing the desired differential pressure. U.S. Pat. No. 5,150,574 dated Sep. 29, 1992 by Toichi Hirata et al. attempts to overcome this problem by providing a counteracting force to the pressure compensating valve that is established by a relationship between the pressures upstream and downstream thereof. This arrangement appears to at least partially offset the flow forces, but due to the pressure acting on cross-sectional areas that are unequal in size, the resulting counteracting force is not directly proportional to the varying difference in the pressure drop across the pressure compensating valve. Furthermore, this counteracting force is, at least in part, always responsive to the highest system pressure. Consequently, its affect on any other circuit in the system that is being loaded at a lesser amount would be receiving a counteracting force to the valving element of the pressure compensating valve that is not typical for that particular circuit.
The present invention is directed to overcoming one or more of the problems as set forth above.